反射震測資料之震波速度與震幅-支距分析推估台灣西南海域地層中天然氣水合物之潛能

Abstract

在台灣西南海域所蒐集到的震測資料發現有廣泛分佈的海底仿擬反射(BSR)，顯示蘊藏豐富之天然氣水合物與游離氣，一旦未來天然氣水合物的開發技術成熟，極可能成為我國主要的能源來源。因此對台灣西南海域天然氣水合物的潛能評估，是需要積極進行的工作。 本研究將地層中出現天然氣水合物地球物理特徵之區域，利用震波速度與振幅隨支距(AVO)分析技術進行其飽和度之估算。由於震波速度對於飽和度之估算極具重要性，因此，獲得正確的震波速度資訊為瞭解沈積物特性之關鍵。為了提升速度計算之精確性，我們利用疊前深度移位技術與疊代移位速度分析方法，在台灣西南外海出現BSRs訊號處，分析所蒐集到之長支距反射資料。研究結果顯示在封閉構造上之BSR處，其上下層速度異常值變化非常大，主要由於其上方蘊藏天然氣水合物造成地層速度值增加，其值增加至1750-2000公尺/秒之範圍，主要分佈在1900公尺/秒附近，相對地，下方由於出現游離氣之分佈使得速度值明顯下降，形成低速帶。此外，由於BSR下方分佈游離氣，造成P波速度與泊松比較上覆沈積物低，AVO屬性分析結果顯示台灣西南外海之BSR屬於第三類型沈積界面(Class-3)，亦即天然氣水合物與游離氣分別存在於BSR之上、下地層中。在飽和度分析中，地層中速度異常值明顯處，其天然氣水合物之飽和度可高達30 %。藉由本研究之成果可提供台灣西南海域天然氣水合物在空間分佈與性質研究上相當寶貴之經驗與資訊，對未來鑽井之井位選擇亦有所助益。

Marine seismic reflection data collected from offshore southwestern Taiwan show that prominent seismic bottom simulating reflectors (BSRs) are presented that indicate the existence of gas hydrate in the seafloor sediment with free gas zone underneath. It is of great interest to estimate the energy resource potential of the gas hydrate in this region. This study intends to derive accurate velocity values and performed amplitude versus offset analysis techniques to estimate the gas hydrate and free gas saturations in the sub-strata where gas hydrate present. The saturation estimates are extremely sensitive to the input velocity values, therefore, accurate velocity determination is crucial for correct reservoir characterization. In order to obtain good velocity values, we apply pre-stack depth migration method to analyze large-offset seismic reflection data where prominent BSRs are observed through the use of iterative migration velocity analysis (residual moveout method). The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and low velocity zones associated with free gas at the location of structural traps. Velocity structures derived from pre-stack depth migration show that the hydrate-bearing sediments generally have velocity ranges from 1750 to 2000 m/s, with most values around 1900 m/s. Low velocity zones observed beneath the gas hydrate bearing sediments clearly indicate the presence of free gas below. For the case of gas-saturated sediment below the methane hydrate layer, Poisson’s ratio and P-wave velocity of the gas-saturated sediment should be lower than the methane hydrate layer, producing Class-3 AVO response. All the seismic characters described above suggest that a gas hydrate layer exists together with a free gas layer below it along strong BSRs in the area offshore southwestern Taiwan, and the saturation of gas hydrate could reach 30% where velocity anomalies are high. In study areas where no well data are available and lithologic heterogeneities are poorly understood, implementing a seismic-based technique like the one presented here can provide valuable pre-drill information for site selection and for planning future characterization studies of gas hydrate-bearing sediments.